In a typical imaging and analysis systems a target is scanned in two dimensions by a probe signal. A corresponding detected signal provides information about the scanned points in the target which can then be displayed as an image and analyzed visually or by electronic processing.
Real-time imaging consists of repeatedly scanning, detecting and displaying the resulting images at a sufficiently high rate to observe changes in the target scanned as the changes actually occur and to avoid motion related analysis issues.
For real-time scanning and display, one dimension of the image must be scanned and displayed at high speed. For example, in a typical video system the high speed dimension is referred to as the horizontal scan rate, while the lower scan rate dimension is referred to as the vertical rate.
Real-time non-invasive imaging is a powerful technique for non-destructive imaging or quantitative analysis of a variety of targets including, semiconductor wafers, materials, human tissue, etc. The analysis can include scanning for defects, discontinuities, or in the case of tissue, abnormalities such as malignant growths.
Real-time imaging is particularly valuable in the case of in vivo analysis of human tissue, where non-destructive, non-invasive sub-surface scanning allows convenient analysis of potentially abnormal tissue without the need for a costly, time consuming and invasive biopsy.
The two dimensional scanning of real-time imaging systems typically consists of a high speed scan along one axis and a low speed scan along an orthogonal direction. A typical video rate imaging system requires a high speed scanning rate of the order of 15 KHz and low speed scan of the order of 30 Hz, providing a progressive scan image rate of 30 images per second. These rates are typically sufficiently high, with respect to the motion speeds, to support in vivo display of tissue without blurring motion artifacts.
There are existing scanning technologies suitable for low speed scanning. These include electromechanical based technologies, such as galvanometers or moving coils actuators. However, these technologies are not suitable for high speed scanning. Existing high speed scanning electromechanical technologies include rotating polygons which are expensive and physically large and in addition have significant alignment issues.
High scanning rates can also be achieved by acousto-optic scanning where an optical beam is deviated by a chirped acoustic wave propagating through a crystal. The acoustic wave is generated by applying an RF signal to the crystal by means of a transducer. The RF signal has a repetitive and linearly varying frequency which provides a matching linearly varying frequency (chirp) to the acoustic wave. The acoustic wave intercepts the optical wave and deviates it by an angle proportional to the RF frequency. This technique allows high speed scanning, however, it is expensive, requires significant RF power because the angular deviation is small, and the system is physically large.
One or more of these aspects of high speed moving parts, high cost components, high power consumption and large physical size make existing scanning systems unsuitable for cost effective, compact, robust, high speed imaging systems.
A typical sub-surface imaging technology, such as confocal microscopy, can generate tomographic images for example of tissue, containing information similar to biopsy sections by scanning a one dimensional array, parallel to the surface of the tissue (x-scan), at varying depths (z-scan) in tissues. The series one dimensional scans at various depths can be displayed as a single tomographic image. Such imaging systems, however, have many of the undesirable aspects described above.
Another sub-surface imaging technology, optical coherence tomography, can also generate tomographic, biopsy-like images. Such systems use a super-luminescence diode (SLD) as the optical source. The SLD output beam has a broad bandwidth and short coherence length. Optical coherence tomography involves splitting the output beam into a probe and reference beam. The probe beam is applied to the system to be analyzed (the target). Light scattered or reflected back from the target is combined with the reference beam to form the measurement signal.
Because of the short coherence length only light that is scattered from a depth within the target such that the total optical path lengths of the probe and reference are equal combine interferometrically. Thus, the interferometric signal provides a measurement of the scattering value at a particular depth within the target. By varying the length of the reference path length, a measurement of the scattering values at various depths can be measured and in this manner, the z-axis can be scanned. The reference path length is typically varied by physically moving a reflecting mirror.
In order to get the biopsy-like image, the second dimension scan (the x-scan) is obtained by translating the probe focusing mirror parallel to the target surface. However, at least some of the above mentioned limitations apply to this imaging method also and, in general, these limitations represent a barrier to applying current imaging technologies to compact, cost effective real-time applications.
There is therefore an unmet need for a cost effective, compact, robust, high speed scanning technology that has no moving parts and that is compatible with imaging systems.